I played with a guy who used a Hiwatt 50 through an Orange 4X12 and wouldn't use an overdrive pedal. He said he had to turn it up to get "that sound." You can get into a volume war with a Hiwatt, but you aren't likely to prevail. I used a 50 watt JCM800 and a 2X12 with JBL E-120s. Won't beat the Hiwatt on dBs, but e-v-e-r-y s-i-n-g-l-e dB is right in the hurty part.

The loudest clean I ever got from an amp without any pedals or mic's, just pure amp was a Dual Showman with 2x15? D130's.
It wasn't my amp and I only got to play around with it about 10 minutes, but I was blown away.
Never could justify owning an amp like that and I guess that was the case for most people because I do not see very much attention to them in the forums.
But if you want endless clean, give the Dual Showman a look.

Also the Super Twins with another pair of 6L6s. Ampeg made a guitar version of the original SVT bass amp. Very rare like maybe just a couple for the Stones but that had to be loud !
I used to have a 50 watt Hiwatt that's probably the loudest amp I've ever owned. You could feel the volume !

Since we're talking Twin Reverbs and loud clean amps, I read a while back that Steve Jones claims all he played with the Sex Pistols was his Les Paul plugged straight into a Twin Reverb - no pedals, just volume.

A TR played loud enough to breakup is pretty loud. Plugged straight in a guitar with hotter pickups than the Fezzsters in my Teles helps to push it, Les Paul or ES-330. (be careful where you stand with a 330)

Since we're talking Twin Reverbs and loud clean amps, I read a while back that Steve Jones claims all he played with the Sex Pistols was his Les Paul plugged straight into a Twin Reverb - no pedals, just volume.

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I had read he had a MXR D+ in front of it, but I'm not disputing your info... for a long time I played an LP or a Tele into a twin with no speakers>1960A. I never turned it up to 10 (small clubs) but used a D+.
Somebody at cbgbs said "hey it's Steve Jones rig!"

There are plenty of amps that are louder than a Twin, SUNN made some very loud clean tube amps, someone already mentioned Hiwatt.
I had a Sound City head with six 6L6 tubes, 120 watts, the old SS Acoustic amps were very loud.
Peavey Roadmaster , Rockmaster, Festival all are loud.

And the Fender 400PS, I've never seen or heard one in person, something that big and heavy just has to be loud!

There are plenty of amps that are louder than a Twin, SUNN made some very loud clean tube amps, someone already mentioned Hiwatt.
I had a Sound City head with six 6L6 tubes, 120 watts, the old SS Acoustic amps were very loud.
Peavey Roadmaster , Rockmaster, Festival all are loud.

And the Fender 400PS, I've never seen or heard one in person, something that big and heavy just has to be loud!

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Leo himself made one - the Music Man HD130. That's a big heavy clean amp right there. Actually, I think Tom Walker was the mastermind behind the amp side of Music Man, but still.